Convolutional neural networks (CNNs) are widely used in geotechnical engineering. Real-time detection in complex geological environments, combined with the strict power constraints of embedded devices, makes Field-Programmable Gate Array (FPGA) platforms ideal for accelerating CNNs. Conventional parallelization strategies in FPGA-based accelerators often result in imbalanced resource utilization and computational inefficiency due to varying kernel sizes. To address this issue, we propose a customized heterogeneous hybrid parallel strategy and refine the bit-splitting approach for Digital Signal Processor (DSP) resources, improving timing performance and reducing Look-Up Table (LUT) consumption. Using this strategy, we deploy the lightweight YOLOv5n network on an FPGA platform, creating a high-speed, low-power subsurface geotechnical defect-detection system. A layer-wise quantization strategy reduces the model size with negligible mean average precision (mAP) loss. Operating at 300 MHz, the system reduces LUT usage by 33%, achieves a peak throughput of 328.25 GOPs in convolutional layers, and an overall throughput of 157.04 GOPs, with a power consumption of 9.4 W and energy efficiency of 16.7 GOPs/W. This implementation demonstrates more balanced performance improvements than existing solutions.